X-ray generating apparatus of the type noted above are disclosed in Japanese Unexamined Patent Publications 2002-25484, 2001-273860 and 2000-306533, for example.
In these apparatus, electrons (Sa [A]) are emitted from an electron source maintained at a high negative potential (−Sv [V]) in a vacuum. Secondly, the electrons are accelerated by a potential difference between the electron source and ground potential 0V. Thirdly, accelerated electrons are converged to a diameter of 20 to 0.1 μm with an electron lens. Finally, the converged electrons collide against a solid target formed of metal (e.g. tungsten (W), molybdenum (Mo) or copper (Cu)), thereby realizing an X-ray source sized in the order of microns. A maximum energy of generated X-rays is Sv [keV].
An especially high-resolution apparatus among these apparatus is called a transmission X-ray generating apparatus or a transmission X-ray tube. Such an apparatus, for example, has a target with a film thickness of about 5 μm formed on a thin aluminum holder (e.g. 0.5 mm thick plate). X-rays generated at the target are transmitted through the holder, in the direction of incident electron beam, and transmitted X-rays are utilized in the atmosphere. The above holder is called a vacuum window, which is used because the thin target in film form is not strong enough to withstand atmospheric pressure. The vacuum window is clamped tight and fixed to a vacuum vessel by an O-ring or the like. This fixing portion is the center of a forward end of an electron lens, and has an evacuated path with a diameter of about 10 mm for converging and passing the electron beam.
In such a transmission X-ray generating apparatus, the target is disposed very close to the electron lens. As for the primary reason, thereby reducing the influence of aberration of the electron lens, and also the diameter of electron convergence is minimized. Thus a minimum X-ray focus is obtained, and high-resolution X-ray fluoroscopic images are realized. As for another reason, thereby the inspection object is close to the X-ray focus, and thus high magnification images obtain. Such an transmission X-ray tube is used in an inspection apparatus for searching for minute defects in an inspection object. These inspecting operations will sometimes take several hours per object. The conventional apparatus constructed as described above has the following drawbacks.
When accelerated electrons (electrical power Sa·Sv [W]) collide with the target, a large part of the electrical power changes into heat, thereby resulting in an X-ray generating efficiency of 1% or less. The heat generated by the electron collision raises the temperature of an electron-colliding portion of the target. Consequently, the temperature raise evaporates the target material and causes various problems.
Thus, the transmission X-ray generating apparatus is halted at the end of target life. The vacuum window clamped to the vacuum vessel is loosened and turned or changed, so that the electron collision portion is replaced to a new target surface. Subsequently the operation of the apparatus is resumed. This causes a problem that X-rays cannot be generated continuously over a long period of time, or a problem of lowering the operating ratio of the X-ray generating apparatus. Particularly where a large object is inspected, the apparatus is operated with an increased load power in order to increase X-ray intensity. In such a case, the life of the target is short and the X-ray generating apparatus must be halted frequently. Further, there is a limit to the X-ray intensity that can be outputted. Since the microfocus X-ray tube is relatively dark, its working throughput cannot be increased.
A method of trial calculations of a target life from electron beam power and a beam diameter is described hereunder.
When an absorbed electric power (Sv·Sa [W]) collides, within a circle of diameter s [μm], with a surface of semi-infinite solid of thermal conductivity K [W/cm° C.], the steady state temperature rise ΔT [° C.] is expressed as follows (reference: Junzo Ishikawa, “Charged Particle Beam Engineering”, Corona Co., May 18, 2001, 1st edition, p145):ΔT[° C.]=2×104·(Sv·Sa)/(πKs)  (1)
This equation (1) shows that the temperature rise is proportional to the electrical power and is inversely proportional  to the collision diameter s. The equation shows also that the temperature rise depends on the electrical power per diameter. Moreover, temperature rise ΔT is inversely proportional to the root of the collision area S, because the collision area S is expressed as π(s/2)2. For example, same electrical power and four times area causes half temperature rise.
When the target is formed of tungsten (W), a trial calculation of ΔT is done by using thermal conductivity K=0.9 [W/cm° C.] at the melting point (3,410° C.) of tungsten. And after the trial calculation, the temperature of collision portion in the target at 27° C. (i.e. at room temperature) is given by a equation, T=300+ΔT [K].
Next, a trial calculation of an amount of evaporation d [kg/m2 sec] of the solid at temperature T [K] is done by the following Langmuir equation (2):d=4.37×10−3·P√(M/T)  (2)
In this equation, M is the atomic weight of a solid material, and that of tungsten is M=183.8. P[Pa] is the vapor pressure of the solid at the temperature T[K] and is derived from the following equation (3):logP=−A/T+B+C logT−DT+2.125  (3)where constants A=44000, B=8.76, C=5 and D=0.
A trial calculation of an amount of evaporation (thickness) per unit time [μm/time] is done by changing the unit of the above amount of evaporation d, thereby dividing by the density of tungsten (19.3 [g/cm2]). Further respecting for a small X-ray focus, the target life is regarded as a time evaporating a thickness corresponding to the collision diameter s.
Results of trial calculations are shown in FIG. 1 under various electron beam conditions and various problems are discussed hereinafter.
Problem 1
“An operating time loss is caused by the target life.”
Load condition No. 1 is an example of ordinary use load of the microfocus X-ray tube. An electron beam power 0.32 W collides with a collision diameter s=1 μm, as a result of a calculation, the temperature of the colliding portion is 2,576K and the life is 142 hours.
In this case, the apparatus is stopped every 142 hours for maintenance work, the vacuum window is loosened and is turned to receive the electron beam on a new target surface. Once loosening the vacuum window breaks the vacuum, and the envelope must be evacuated again for about two hour. Then the operation is resumed. Thus, X-rays cannot be generated for about two hours, and hence there is a problem of lowering the operating ratio of the apparatus. Consequently maintenance work has to be done for two hours once a week, and this operating ratio is 142/(142+2)=99% for assuming a continuous operation. In some case the life will be extended by lowering the power, however reducing X-ray intensity and requiring a longer time for fluoroscopy, thereby working throughput will reduce.
Problem 2
“There is an upper limit to X-ray intensity, and no improvement in working throughput.”
Load condition No. 2 is an example in which X-ray intensity is slightly higher than the loading condition No. 1. The current is increases by 9% with the same acceleration voltage, and also the electron beam power increases by 9% from 0.32 W to 0.35 W. Thus, X-ray intensity increases by 9% and working throughput also by 9%. However, as a result of a calculation, the temperature of the colliding portion is 2,790K and the life is calculated to be seven hours. In this case, the mere 9% increases in X-ray intensity results to stop the apparatus every seven hours for maintenance work. The operating ratio of the apparatus falls off to 7/(7+2)=78%.
Load conditions No. 3 and No. 4 are examples where X-ray intensity is about three times that of load condition No. 1. As a result of the trial calculations, the temperature of the colliding portion exceeds the fusing point (about 3,680K) and boiling point (about 6,200K) of tungsten. Since the target material evaporates quickly, these conditions are impracticable. If X-ray intensity were increased by three times, working throughput would be three times higher since the time required for generating the same X-ray dosage would be one third. Consequently, there is a limit to load power and an upper limit to X-ray intensity, hence working throughput cannot be improved.
Problem 3
“The tube is darkened by minute focusing.”
Temperature rise ΔT is dependent on the electron beam power per diameter as expressed by equation (1). Therefore, when the electron beam is narrowed down to reduce the collision diameter, the the electron beam power must also be reduced. Assume, for example, a case where the collision diameter s=0.1 μm to secure a minute X-ray focus for higher resolution. Since power must be reduced to one tenth in order to obtain the same evaporation rate as in load condition No. 1, X-ray intensity also becomes one tenth and working throughput one tenth. Moreover, since the life is determined by “Further respecting for a small X-ray focus, the target life is regarded as a time evaporating a thickness corresponding to the collision diameter s”, the evaporating thickness to the end of life is one tenth, and life is reduced to one tenth, i.e. 14.2 hours. The operating ratio of the apparatus decrease to 14.2/(14.2+2)=88%. Such minute focusing is needed in order to cope with the micro-fabrication of integrated circuits in the semiconductor field today, and therefore is all the more problematic. Load condition No. 5 is a desirable example in which the collision diameter s=0.1 μm and the electrical power is set to 0.24 W which is 75% of the load condition No. 1. As a result of the trial calculations, the temperature of the colliding portion is 1,7371K, and the quick evaporation makes this condition impracticable.
Problem 4
“Caution is needed because of delicate changes in focus shape.”
When X-ray irradiation is carried out continuously for 142 hours with the load condition No. 1 in FIG. 1, the target becomes thin as a result of the 1 μm evaporation. During this evaporation, the shape of the target surface struck by the electron beam varies, and the shape and position of the X-ray focus undergo delicate changes. Since a microfocus X-ray apparatus is required to keep high spatial resolution, a fine adjustment of the electron beam is needed even within the lifetime. Therefore, this reduces the operating ratio of the apparatus. Moreover, it should be noted that the life shown in FIG. 1 is tentative and not absolute.
Problem 5
“A thick target unnecessarily absorbs X-rays.”
In order to provide a similar X-ray intensity during a life, the target should have a thickness at least equal to a sum of a maximum depth of electron penetration and a thickness corresponding to the target life. Also in order to withstand power increases due to voltage variations or the like, the target usually is formed somewhat thick.
For example, accelerated electrons with an energy of 40 keV at the time of a 40 kV tube voltage collide with the tungsten target and enter the target by a maximum depth of 2.6 μm while generating X-rays of 40 keV or less. Thus, for the 40 kV tube voltage and 1 μm collision diameter, a target thickness of at least 3.6 μm is needed, and a thickness of about 5 μm is adopted to allow for a margin.
However, since the maximum depth of the X-ray generating region is 2.6 μm, only the X-rays not absorbed by the remaining 2.4 μm of the target thickness of 5 μm is used as transmitted X-rays. This constitutes a low utilization rate of the generated X-rays. Where, for example, X-rays of 20 KeV pass through the tungsten of 2.4 μm, only 80% is transmitted. Thus, X-ray intensity is low and the working throughput falls off to 80%.
Problem 6
“A rotating anode X-ray tube is incapable of high resolution.”
To solve the problem caused by the heat of the target, an X-ray generating apparatus of millimeter-size focus for medical use employs the rotating anode type. However, rotational accuracy is insufficient with a bearing (ball bearing) used for rotation, and the anode target is not rotated with high accuracy, then the X-ray focus is blurred. Therefore the rotating target is difficult to apply particularly to the microfocus X-ray generating apparatus having an X-ray focal size in the order of microns. The above problem is discussed more particularly hereinafter.
The rotating anode X-ray tube has an X-ray focal size in the order of 0.2 to 1 mm, and has a vacuum vessel, an electron source, an anode disk, a rotating bearing and a motor formed as an integrated unit. But the motor is spaced from the electron beam, because the motor generating an electromagnetic force deflects the electron beam unnecessarily. Thus, the rotating anode X-ray tube tends to be large. Further, a ball bearing is employed as the rotating part and has an inside diameter of 6 to 10 mm, an outside diameter of 10 to 30 mm or more, and a thickness of 2.5 to 10 mm or more. The highest accuracy class of ball bearings in this range of sizes is specified in Class 2 of the Japanese Industrial Standards, and the axial deflection accuracy and radial deflection accuracy of the inner ring are as much as a maximum of 1.5 μm. Since the X-ray tube is used in severe conditions of high vacuum, high temperature and high speed, a special lubricating system is used. The degree of vacuum inside the X-ray tube, for example, has to be 0.13 mPa (10−6 Torr) or less. The bearing is operable in the temperature range of 200 to 500° C. due to the generating heat of the anode, and a high-speed rotation in the order of 3,000 to 10,000 rpm (50 to 167 cyc/sec) is also required. In order to satisfy such severe conditions, the X-ray tube employs a very special bearing using a thin coating of soft metal as solid lubricant. However, since the life of the solid lubricant is short, the life of the rotating anode X-ray tube also has a life of only several hundred hours.
The microfocus X-ray tube has a lower load power than the X-ray tube for medical, therefore the target holder does not reach such a high temperature. However, bearing steel has a coefficient of linear thermal expansion in the order of 12.5×10−6 (1/° C.), and a temperature rise of only 20° C. lowers its rotational accuracy with the inside diameter expansion of 1.5 to 2.5 μm. A temperature rise of about 20° C. easily occurs with a change in a room temperature or with a heat generated by rotation friction. Combined with the rotational accuracy specified in Class 2 of the JIS, a rotational accuracy of 3 μm or less is unwarranted and impracticable. Further, the rotating anode disk have a diameter of 10 mm or larger because of the outside diameter of the bearing, and the whole waviness of the target surface, since tungsten is extremely hard and difficult to shape, varies the X-ray focal position by about 10 μm. Accuracy of this level is not problematic with the medical X-ray tube whose X-ray focal size is about 0.2 to 1 mm. However, with the microfocus X-ray tube whose X-ray focal size is in the order of microns, focal size variations and focal position shift in the electron beam directions make the application of the rotating anode type difficult.
The bearing is at least five times thicker than the transmitted X-ray type vacuum window which is about 0.5 mm thick, whereby the rotating anode type has to be large. The rotating anode requires a vacuum window as an essential component for acquiring X-rays. That is, the rotating anode and an object under inspection cannot be brought close to each other, and it is accordingly difficult to increase geometric magnification. Even if a high-accuracy ball bearing is developed, it will be difficult to obtain high-resolution X-ray fluoroscopic images.